Graphic display devices using liquid-crystal technology are being used increasingly to display information in motor vehicles. Because of the high optical demands, so-called "active displays" with internal color filters are used with transparent lighting. Powerful back-lighting sources with adjustable luminance are required for this purpose.
Various requirements are imposed on these light sources. In addition to uniform illumination and high electro-optical efficiency, low heat emission and low temperature dependence of the luminance are required. Because of the considerable changes that occur in the lighting conditions in a motor vehicle, a luminance which can be controlled over a wide range is required. The minimum luminance must be less than 100 cd/m.sup.2 and the maximum luminance must be greater than 25,000 cd/m.sup.2. Finally, white light must be radiated over the entire dimming range.
Because of their efficiency and associated low heat emission, as well as their flat design, fluorescent lamps, especially compact fluorescent lamps, are suitable for this purpose. However, there are difficulties relating to the broad dimming range and temperature independence.
The goal of the present invention is to provide a circuit for operating a fluorescent lamp which permits fulfillment of the above requirements.